Thanks to all those who responded with suggestions. The solution was.....
I edited /etc/X11/xorg.conf on the client and:
Added>>>>>>>>> this section
Section "Monitor"
Identifier "Monitor0"
HorizSync 31.5-62.5
VertRefresh 50-70
EndSection
In the "Screen" secion:
Added Monitor "Monitor0"
In the display subsection:
Added/revised Modes "1280x800" "1024x768" "800x600"
After a reboot of the client, the screen resolution was 1280x800, which is fine and the GUI pull-down list of resolutions included a range of options.
Brendan
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Jerry James wrote:

2009/12/11 Brendan Whelan <b_whelan mistral co uk>:

The only options in the guest are 640x480 and 800x600.
I don't know where the list is stored where I could add another set of
values.
Brendan